Separating oil and water in oil/water mixtures is often desirable. Furthermore, since the implementation of environmental and water-quality regulations, it is often necessary to achieve reduction of oil concentration to less than 50 ppm. There are presently several separation systems available for separating oil from water.
One simple separator system comprises a settling basin in which oil and water separate over time by gravity due to their density differences. The degree of separation is directly related to residence time in the basin.
Another method which is known as floatation uses the buoyancy of gas bubbles rising through the liquid to "float" contaminants, such as oil droplets, to the surface. The gas bubbles may be formed by the bubbling out of dissolved gas that occurs when pressure on the system is reduced or by injecting or dispersing gas into the water by a bubbling device.
Hydrocyclones are known separation devices which use a centrifugal effect to cnhance separation. One design of a hydrocyclone comprises a long, funnel-shaped chamber into which a feed line is tangentially directed. An oil and water mixture under pressure is directed tangentially into the funnel-shaped separation chamber of a hydrocyclone via the feed line whereupon its energy is converted to angular momentum as the mixture swirls around the inside of the chamber. The swirling causes the less dense portion of the mixture (the oil) to move towards the axis of the device while the more dense portion (the water) is urged to the outside.
A typical hydrocyclone has a coaxial overflow outlet in its large end for providing an outlet for less dense phase from the hydrocyclone, and a coaxial underflow outlet, at the opposite end, for providing an outlet for the more dense phase from the hydrocyclone. The pressure difference between the overflow outlet and the underflow outlet, and the inlet flow rate determine the relative volumes of the overflow and underflow streams. Increasing the pressure at either outlet causes the flow through the opposite end to increase.
A predetermined degree of separation for a particular feed is achieved by providing a high enough velocity to create sufficient centrigual force and by setting the relative volumes of the overflow and underflow. The pressure differential between inlet pressure and overflow pressure necessary to achieve sufficient overflow rate in a given hydrocyclone at a given inlet flowrate can be calculated. The pressure at the underflow outlet must be greater than the pressure at the overflow outlet to provide sufficient overflow rate.
It has been demonstrated that the oil droplets that remain in the water underflow of a hydrocyclone are coalesced to droplets having a large size by the action of the hydrocyclone. Because the larger droplets settle out of the water at a higher velocity than the smaller droplets, it is easier to separate in a skim vessel the oil remaining in the hydrocyclone underflow from the water than it is to separate the smaller oil droplets from the water in the inlet stream.
U.S. Pat. No. 5,021,165 discloses a system having separator vessels and a hydrocyclone for separating oil from water that includes a pressure reducing device immediately downstream of the hydrocyclone. The pressure reducing device allows the necessary pressure to be maintained at the underflow outlet while providing a reduction of pressure to vaporize part of the stream to "float" oil droplets in a flotation unit downstream of the hydrocyclone. The passage of the fluid through the pressure reducing device, however, causes shearing of the oil droplets in the water thereby significantly decreasing the efficiency of the flotation unit.
In any mixture of immiscible fluids, because the kinetic energy of the mixture contributes both to dispersion of larger droplets and coalescence of smaller droplets, at any given energy input rate there is a statistically defined maximum droplet size for which the rates of dispersion and coalescence are equal. Maximum droplet size is inversely related to the energy input rate of the system. It is also known that a rapid decrease in pressure results in shear forces on the mixture causing shearing of oil droplets larger than a certain diameter. Hence, to maintain the large droplets developed by coalescence in the hydrocyclone, it is desirable to minimize the pressure drop to which the mixture is subjected.
One disadvantage of the prior art is that the use of pressure reduction to effect flotation causes a decrease in the efficiency of the separation for the reasons stated above. According to the present invention, a mixture comprising at least two immiscible liquids is processed through a system to separate the liquids by avoiding pressure reduction except for the pressure reduction in the hydrocyclone itself. The avoidance of the pressure reduction reduces the difficulties in shearing caused by such reduction in the prior art.
These and other objects and advantages of the present invention will be apparent from the following description.